British lawyers attempting to build a war crimes case against Israel have been blocked from entering the Gaza Strip because the Foreign Office has refused to support their work, the Guardian has learned.

A number of the lawyers, who have travelled to the region on fact-finding trips, could not get into the Palestinian territory because they cannot cross the border without letters from the British government authorising their visits.

One lawyer, whose MP, Diane Abbott, wrote to the Foreign Office on her behalf, was told her effort would be better spent contributing to humanitarian work.

"I recommend that Ms Maynard get in touch with the UN ... to enquire how she might best be able to assist the international humanitarian effort," Bill Rammell, a Foreign Office minister, told lawyer Kate Maynard, in a letter seen by the Guardian.

The letter confirms the government has "consistently refused to issue letter requesting facilitation to enter Gaza".

"This seems like a determined effort not to enable important witnesses to get into the Gaza Strip," said Daniel Machover, senior lawyer at London firm Hickman & Rose, who is working on cases of alleged war crimes in Gaza. "No other European country has adopted this stance. That is what is so incredible about this."

"All the letter states is that the Foreign Office has advised you not to enter Gaza and you have not accepted its advice," said Stephen Kamlish QC, who was recently refused a letter by the British embassy in Cairo. "But you have to have one to get across the border." Kamlish, who later entered Gaza with a group of French officials, is one of a growing number of British lawyers building a case against individuals within the Israeli government and military for war crimes and breaches of international humanitarian law.

But a number of lawyers, operating independently or through organisations such as the UK-based Lawyers for Palestinian Human Rights, have had to turn back after waiting for days at the Rafah crossing in Egypt because of the Foreign Office's refusal to provide the official documentation. Others have been forced to rely on sources inside Gaza or Israel to provide sponsorship letters. "We are just not getting any response from the Foreign Office since the events in Gaza," said Rashad Yaqoob, a lawyer for the Human Rights Legal Aid Trust, which is funding the work of human rights lawyers in Gaza.

In February lawyers representing Palestinian charity al-Haq brought a claim against the British government at the high court, claiming it had breached its obligations under international law by failing to respond to Israel's action in the Gaza Strip. "In my opinion the Foreign Office is now under pressure from the judicial review ..." Yaqoob said. "I don't see them as being too comfortable in facilitating lawyers going across where we could be collecting evidence that could be used against them."